I guess I'm gonna fire an other question back at you: The first thing I'd be doing is wondering why they are all sporting messy bottoms. More often than not, it's a sign of vent gleet: In that case, the first thing I'd be doing is looking at their environment. Do they get to free range on a daily basis for extended periods of time? If not, how big is their run, and what is it like? How many square feet per bird? Is it covered with grass, other green matter? Is it bare soil?
Vent gleet is caused by their intestinal flora being out of balance. You can help them out by giving them plenty of free range time, giving them a run that is not bare soil. This is most easily accomplished (assuming that you don't have plenty of daily free range available, and that you don't have a huge run consisting of about 50 - 100 sq. ft. per bird) by laying down a deep litter in the run (aim for at least 4" deep, 6" is better. This will give them plenty of beneficial bacteria and fungi to help get their guts back into good shape. As a bare minimum, they should have 10 sq. ft. in the run per bird.
The next thing you can do, and I consider it to be of equal importance: Give them fermented feed. I'm looking for the article that gives you lots of info about the why and the how, but in the mean time, you can get started by doing a thread search. Fermented feed gives your birds lots of healthy probiotics (similar to comparing yogurt to pasteurized milk). It breaks down the antinutrients in the grains, making them easier to digest, so they absorb more nutrient from each scoop of feed. It causes their guts to actually become more healthy with increased villi.
Finally, if changing up their environment and their feed doesn't show improvement after a couple of weeks, I'd suggest you take some fecal samples to your vet and have a fecal float test done. I'd not simply throw worm medication at them, because that may not even be an issue, and if you worm them, you will be adding insult to injury to their digestive systems that are already stressed.
